The invention herein described was made in the course of or under a contract or subcontract with the Department of the Navy.
1. Field of the Invention
This invention relates to the field of nickel base superalloy articles used where high mechanical stresses are encountered at high temperatures. This invention is also related to the field of directionally solidified eutectics wherein an alloy of approximately eutectic composition may be directionally solidified so as to produce an aligned multiphase microstructure having anisotropic mechanical properties. The article of the present invention is comprised of a directionally solidified nickel base superalloy of approximately gamma prime-alpha (Mo) eutectic composition having exceptional mechanical properties combined with adequate oxidation and sulfidation resistance at elevated temperatures.
2. Description of the Prior Art
U.S. Pat. No. 2,542,962 to Kinsey describes a broad range of compositions in the aluminum, molybdenum, nickel system without mentioning eutectic compositions or directional solidification. It is now known that certain eutectic alloys respond to proper directional solidification conditions to produce useful second phase aligned microstructures as described in the patent to Kraft, U.S. Pat. No. 3,124,452. In a patent to Thompson, U.S. Pat. No. 3,554,817, there is described a promising pseudo binary eutectic alloy occurring between the inter-metallic compounds Ni.sub.3 Al and Ni.sub.3 Cb which responds to plane front solidification to produce a casting characterized by an aligned lamellar microstructure. As so solidified, this combination provides one of the strongest nickel base alloys known although its ductility is limited.
In a prior patent to Thompson and Lemkey, U.S. Pat. No. 3,564,940, there is described a class of compositions which solidify according to the monovariant eutectic reaction providing aligned polyphase structures including such systems as the ternary alloys identified as cobalt-chromium-carbon and nickel-aluminum-chromium. The advantage of compositions of this nature is that the desired microstructure can be achieved over a range of compositions in a given system. This provides a substantial increase in the freedom of selection of compositions permitting increased optimization of properties. In U.S. Pat. No. 3,671,223, the concept has been further developed to include those systems which solidify according to the multivariant eutectic reaction where two or more solid phases (N) crystallize simultaneously from the liquid consisting of (N+2) or more components.
U.S. Pat. No. 3,617,397 to Maxwell, assigned to the present assignee discloses a nickel base superalloy which contains 8 percent aluminum and 18 percent molybdenum. This alloy is far from the eutectic point, and the patent does not disclose the benefits available in the directionally solidified eutectic.
U.S. Pat. No. 3,793,010 to Lemkey and Thompson discloses a eutectic article which consists of a gamma-gamma prime matrix with an aligned delta second phase.
U.S. Pat. No. 4,012,241 to Lemeky discloses a monovariant eutectic which is related to the present invention. The nominal composition of this eutectic is 8% aluminum, 27% molybdenum, balance essentially nickel. This composition is preferably directionally solidified to produce a microstructure consisting of a gamma prime matrix containing aligned fibers of alpha molybdenum and preferably small precipitate particles of the gamma phase in the matrix.
U.S. Pat. No. 4,111,723 to Lemkey, assigned to the present assignee, discloses another extended monovariant eutectic based on the nickel-aluminum-molybdenum system. The nominal composition of this alloy is 4.6% aluminum, 35.8% molybdenum, balance essentially nickel. Upon directional solidification, microstructure will consist of a gamma matrix containing aligned alpha molybdenum fibers and preferably also containing precipitate particles of the gamma prime phase in the matrix.
U.S. Pat. No. 3,904,403 discloses a composition in the nickel-aluminum-molybdenum system and suggests the possibility of directional solidification. Additions of up to 3 atomic percent tantalum are suggested.
The most accurate previous work on the nickel-aluminum-molybdenum alloys is described in "The Form of the Equilibrium Diagrams of Ni-NiAl-Mo Alloys," Academy of Sciences USSR, 132, May-June 1960, pp. 491-495, however, this reference does not discuss the eutectic reaction relied upon in the present invention.
The technical article entitled "Nickel-Aluminum-Molybdenum Alloys for Service at Elevated Temperatures," found in ASM Transactions, Vol. 43, pp. 193-225 (1951) by Kinsey and Stewart, describes work connected with the previously mentioned U.S. Pat. No. 2,542,962.